The present invention relates to an apparatus for detecting the position of label stock for printing. More particularly, this invention relates to a printer for labels having a reflective sensor for detecting the position of an indicator stripe on the label stock for the purpose of aligning the label with the print head, in which an emitter and a detector are mounted on the back side of the label stock and are mounted remotely from a removable cassette having a concave mirror, in such a way that no electrical connections to the cassette are required for sensing the label position.
It is known to print label stock to produce individual labels to mark food products for example, with their description, weight, nutritional information, etc. Traditional optical means of detecting the position of the labels include a "through beam" system wherein an emitter is placed on one side of the label stock and a detector is placed on the reverse side of the label stock. There are two methods for using through beam technology- gap or stripe indication. In the first system, the gap between the labels is sensed as a change in transmitted light intensity. The disadvantage of this approach is that the sensors may detect false gaps depending on liner quality, paper variations, and the presence of pre-printed information on the label.
In the stripe method, a black stripe is printed on either the front or the back of the label supply. When the indicator stripe is present, the light from the emitter does not pass through the labels and is not detected by the detector. The presence of a black bar provides a more definitive transition and is more reliable than the gap method. However, this method is still subject to errors due to the presence of certain pre-printed information. This therefore, restricts the position of the pre-printed information as well as the design and appearance of the labels. The appearance is also affected by the black bar on the face of the label seen by the consumer.
One example of a printer which employs a "through beam" detector is shown in U.S. Pat. No. 5,336,003 to Tokyo Electric Co. This printer employs a prism on one side of the label stock to conduct the light emitted from an emitter to another prism located on the other side of the label stock. A gap between the labels is sensed as disclosed above. This system suffers from the problems previously mentioned from positioning the emitter and detector on opposite sides of the label stock. Also, this system uses a cassette. It is not practical to put electrical connections on a cassette to accomplish the detection, since the electrical connections cannot be simply, reliably and repeatedly made to the cassette. This need to produce the electronics on each cassette greatly hampers the manufacturability and reliability in use and greatly increases the cost of each cassette.
Therefore, it is an object of the present invention to provide a printing apparatus in which the emitter and the detector are on the same side of the label stock and in which a concave mirror mounted on a removable cassette is present to focus and reflect the light being detected to signal that a label is in position for printing. It is a further object of the present invention to provide a printing apparatus in which the labels are supplied from a removable cassette which is free of electrical components.